Process of fluidized bed reduction of iron ore followed by electric furnace melting



. July 14, 1959 5 5,0143 ETAL Z, 8 94,831

PROCESS OF FLU'IDIZED BED REDUCTION OF IRON ORE FOLLOWED BY ELECTRICFURNACEMELTING v Filed Nov. 28, 1956 v 2 Sheets-Sheet '1 a REDUCINGINVENTORS BEUCE SCOTT OLD EICHAED' WARREN HYDE THEIR ATEORNEYS July. 14,1959 OLD ET AL PROCESS OF FLUIDIZED BED REDUCTION OF IRON ORE FOLLOWEDBY ELECTRIC FURNACE MELTING Filed NOV. 28, 1956 2 Sheets-Sheet 2 Fla. 2

IN V EN TORS w DY L J T m ww w wA UH RN B2 TH EIR ATTORNEYS UnitedStates Patent (It 2,894,831 Patented July 14, 1959 ice PROCESS OFFLUIDIZED BED REDUCTION OF RON ORE FGLIJOWE'D BY ELECTRIC FURNACEMELTING Bruce Scott 01d, Concord, and Richard Warren Hyde, Lexington,Mass.

Application November 28, 1956, Serial No. 624,861

9 Claims. (CI. 75-43) This invention relates to the production of ametal from a finely divided reducible compound thereof. Morespecifically, this invention relates to a process for the production ofmolten iron which consists essentially of reduction of iron ore in afluidized bed followed by melting the reduced powdered iron in anelectric furnace. The process of this invention advantageously forms onestep in the production of steel in areas where generated or natural gasfuels and inexpensive electric power are available, but wheremetallurgical grade coking coals are unavailable.

In conventional steel-making practice, iron ore is first reduced in theblast furnace to produce pig iron which subsequently, along with scrap,becomes the charge to the steel-making furnace, namely, the open-hearth,Bessemer, or electric furnace, to produce various grades of finishedsteel. Successful operation of the conventional blast furnace is lareglydependent upon the mechanical strength of the fuel. In the blastfurnace, coke serves as the fuel, reducing agent, and most importantsupports the burden and maintains it permeable to the passage of gas inthe higher temperature zones of the furnace. Thus, metallurgical cokemust be of adequate size and have suflicient strength to resistbreakdown in the furnace. With the growing shortages of high grademetallurgical coke, efforts have been made in recent years to develop areducing process which will not be so particular with respect to thephysical properties of the reducing material. Reduction of iron ore hasbeen tried in various types of equipment such as rotating kilns,Herreshoff roasters, various types of conveyors, crucibles, shaftfurnaces, and even so-called bubble hearth roasters. None of thesesocalled direct or alternative reduction methods have been economicallysuccessful due to the fact that in all cases the nature of the producingunit limits the producing capacity. This means that in order to obtainan equivalent producing capacity, multiple producing units are requiredwhich results in high investment and operating costs, as compared withthe conventional blast furnace. In the steel industry small operationsmean high costs and if an alternative process is to be successful, it isnecessary to develop a process and operating unit which will beequivalent in terms of capacity and cost with the coke blast furnace.Such a method is the subject of this invention.

The typical steel-making process in which the method of the presentinvention is an important step involves reduction of iron ore in orderto produce molten iron by the method of this invention, followed byrefining of the molten iron so produced in steel-making furnaces such asthe open-hearth, Bessemer types, or electric furnace to produce steel.

In order to carry out this invention, the ore is first subjected togaseous reduction in a fluidized bed with generated reducing gas orpartially combusted natural gas or other hydrocarbons used as thereducing agent. Iron ore of the proper particle size distributionsuitable for operation in a fluidized bed is first charged continuouslyto a preheating unit where it is heated by combustion of a portion ofthe top gas from the reducing bed. This preheating unit mayadvantageously be a fluidized bed, but may be any other suitablepreheating unit such as a rotary kiln or a device similar to aHerresholf roaster. By this procedure the temperature of the ore israised, preferably to about the operating temperature of the subsequentreducing fluidized bed. The preheated ore will overflow continuouslyfrom the preheating unit into the reducing fluidized bed where itcontacts hot reducing gases.

The fluidized bed is established by blowing up through the bed a streamof reducing gases derived from burning with air or oxygen either liquidor gaseous fuels including those derived from solid fuels, such asproducer gas. The velocity of the gas stream is controlled withreference to particle size of the ore or reduced material to set up afluidized bed condition in which the particles are dispersed orsuspended in the gas stream without excessive entrainment and carry-overof the solid particles in the gases leaving the zone of solidsuspension. The preheated iron ore will be reduced in the solids statein the reducing bed to the extent that approximately 90% of the ironcontent of the original ore will be reduced to iron while the remaining10-20% of the original iron content will be reduced to FeO. The reducedore will be discharged continuously by overflow from the fluidizedreducing bed. This reduced ore will be finely divided and at elevatedtemperatures highly pyrophoric.

This pyrophoric material discharging from the reducing bed must now beconsolidated, which is carried out in accordance with the presentinvention by converting it to molten iron. In the preferred method ofoperation of this invention this finely divided reduced ore isdischarged continuously into an electric arc melting furnace similar tothe type used in Scandinavian countries to smelt iron ore directly tomolten pig iron. The electric melting furnace is preferably operatedfull and designed to facilitate movement of the charge, in order to feedand maintain a high load factor on the electrodes. It will continuouslyreceive reducing material which will be melted and the impuritiesslagged with suitable fluxing agents. The slag and molten iron will beperiodically tapped from the furnace into ladles. The molten iron maythen be refined by conventional methods to produce steel.

This invention will now be described in more detail in connection withthe accompanying drawings, in which:

Fig. 1 represents a vertical view, partly in section, of suitableequipment for carrying out the procedure of this invention; and

Fig. 2 represents a vertical view, partly in section, of an alternativearrangement of part of the equipment shown in Fig. 1.

The equipment comprises three principal parts: the preheating unit 10,the fluidized reducing bed 12, (both shown in Fig. 1) and theelectric-arc melting furnace 1-4 (shown in Figs. 1 and 2). Associatedwith these parts are suitable ducts for feeding materials to variousparts of the equipment and for leading products away, and also suitableseparators for removing solids from gases. These are described below inconnection with the description of the operation of this invention. Likenumbers refer to like parts in the two figures.

, In carrying out the procedure of this invention, the ore feed materialis first screened or ground to establish particle size distributionsuitable for operation of a fluidized bed. Although iron ores are theones with which this invention is particularly concerned, and which willbe referred to in the further description of this invention, it is to beunderstood that other ores which, when reduced to the metal, exhibitpyrophoric properties when hot and finely divided, are amenable to thetreatment herein described. Such other ores include, for example, otherferruginous ores, e.g., titaniferrous magnetites.

The ore feed material is sized as mentioned above at about minus 8 meshand with a minimum of very fine material (i.e. minus 325 mesh), in orderto minimize the tendency to carry over very fine material in the exitgas stream from the bed. This sized ore is dumped into bin 20 whence itflows via conduit 22. to preheating unit 10. In this unit it ispreheated by the combustion of a portion of the low Btu. top gas fromthe subsequent fluidized reducing bed unit 12. This top gas enterschamber 24 from duct 26, and passes up through distributing grate 28into combustion chamber 30 where it burns in combination with airintroduced through duct 32.. Hot combustion gases rise through grate 34,and heat and agitate the ore particles in chamber 36. These preheatedore particles then pass out through conduit 38. It will be noted thatthis is essentially a fluidized bed preheating treatment; however, otherpreheating methods and apparatus may be used within the scope of thisinvention.

The ore particles passing out through conduit 38 emerge into reducingbed 12, where they contact reducing gas generated from the partialcombustion of natural gas or other fuel introduced through conduit 40and mixed with air which is introduced through conduit 42. The fuel issubjected to partial combustion with air in chamber 44, and theresulting reducing gas passes up through the perforated grating 46 toprovide fluidized reducing Zone 48.

Additional particles are fed to Zone 48 via duct 50 lead ing fromcyclonic separator 52, into which the off-gases from preheating unitdischarge via duct 54.

Gases discharged from bed 12 pass off via duct 56 to cyclonic separator58, whence the separated particles pass downwardly toward meltingfurnace 14 and the cleaned gases pass up through duct 60. A portion ofthese gases passes from duct 60 into line 26 controlled by valve 62, forpreheating in unit 10 as already described. The balance of these gasesis discharged through duct 64 controlled by valve 66.

In the operation of the fluidized reducing bed 12, the individualparticles are in a fluidized state of suspension in the gas emergingfrom chamber 44. In this state the particles are subject to violentagitation and extremely intimate mixing so that for all practicalpurposes the composition of the bed is identical throughout. The gasvelocity must be maintained at the proper level to fluidize the materialand yet not carry over excessive quantities of finely divided materialin the oif-gas. The solid, finely divided ore fed to the bed via conduit38 is removed continuously by overflow through conduit 68. The ore whenproperly fluidized may be handled in a manner similar to handling aliquid. The operating temperature in zone 48 must be sufficiently highto give adequate reaction rates. However, the temperature must bemaintained below the fusion point of the reduced iron particles so thatthe particles do not weld together and change the fiuidizingcharacteristics of the bed material. The chemical equilibriumstoichiometric factors must be such that enough reducing gas is producedto allow the removal of the oxygen as carbon dioxide and/ or watervapor. The thermal requirements of the process must also be met so thatthe heat required by the reduction is supplied by preheating thereducing gas or by adequate partial combustion of the fuel in the bed orjust prior to entry into the bed. Thus, in order to satisfy the thermalrequirements of the integrated process, it will be preferable to preheatthe ore, as in unit 10, by combustion of the portion of the ofi-gas fromthe fluidized reducing bed and also to preheat the air and combustiongases prior to entering the reducing bed. This is done in order toprovide sufiicient heat to meet the thermal requirements of thereduction operation, and maintain the proper temperature level in thereducing bed.

The melting of this highly pyrophoric reduced material emerging from thefluidized bed via conduit 68 is a critical step in the process of thisinvention. The finely divided reduced ore is of no significance and isof little use in the steel industry in powdered form, and, therefore,must be agglomerated preferably to molten iron to complete theprocessing to steel. Although various methods having been proposed formelting this finely divided pyrophoric reduced ore, the electric-oresmelting furnace herein described oifers the best possibilities. if sucha furnace is suitably designed so that a head of material was providedto force-feed the arcs, as shown in Fig. 1, the finely divided reducedmaterial can be melted directly in such an electric-arc furnace 14.Alternatively, as shown in Fig. 2, the reduced material can be fed by aplurality of chutes to the electric-arc furnace 14, using an arrangementgenerally similar to that of a standard Tysland-Hole type electricsmelting furnace. It is interesting to note than an 18,000 kva. Tysland-Hole type electric smelting furnace operating on high grade ore canproduce about 185 tons per day of pig iron. Based on detailed heatbalance calculations for this operation, the operation of thisinvention, it is found that if the reduced ore emerging from conduit 68is substituted in the same size 18,000 kva. Tysland-Hole furnace, about700 tons of pig iron per day can be produced.

The finely divided reduced material discharged from conduit 68, as wellas from separator 58, into electric melting furnace 14 is primarily ironwith a minor amount of ferrous oxide, FeO, in the case of iron ore feedat bin 20. More specifically, about to percent of the iron oxide in theore is reduced to iron, while the remaining 10 to 20 percent is reducedto FeO. The furnace 14, in the embodiment shown in Fig. l, is providedwith a stack 70 high enough to provide a head of material adequate toforce-feed the electrodes 72. Preferably the walls of stack 70 divergedownwardly, to minimize any tendency of the particles to stick to thewalls. Bypass line 73 is provided to conduct waste gases from furnace 14back to separator 58.

Another arrangement for avoiding such sticking is to use multiple chutes74 as shown in Fig. 2. These may, be fed from a circular hopper 76 whichis provided with any suitable distributing means for spreading thematerial around the circumference of hopper 76, e.g. a rotarydistributor or a monorail conveyor, and which is fed from conduit 68.Since these chutes 74 are not pointed directly at the electrodes 72, theparticles in the chutes tend not to be heated to the sticking point byheat from the electrodes.

In either the arrangement of Fig. 1 or that of Fig. 2 the reducedmaterial passes directly to the closed electric furnace from conduit 68,without contact with the atmosphere. Such contact must be avoided, dueto the pyrophoric property of the hot particles.

Molten iron and slag are produced continously in furnace 14, and aredrawn oif from the bottom thereof through openings 80 in the mannerwhich is customary for tapping conventional electric smelting furnaces.

With a fluidized bed 48 which is 20 feet in diameter, discharging intoan 18,000 kva. electric-arc furnace 14, about 500 tons of molten ironper day can be produced using a high grade of iron ore supply. Hence,this combination as herein described is capable of duplicating thecapacity of a 500 ton per day conventional blast furnace, without thenecessity for using the high-grade metallurgical coke which is nowrequired in blast furnace practice.

We claim:

1. An apparatus for producing metal from finely divided reduciblematerials comprising a shaft having a first combustion chamber and areaction chamber therein above said combustion chamber, a firstpartition having passages therethrough interposed between saidcombustion chamber and said reaction chamber, means for introducing acombustion supporting gas and combustible gas into said combustionchamber for partial combustion therein and flow through said partitioninto said reaction chamber to fluidize a reducible material therein andat least partially reduce it to metal, said shaft having a secondcombustion chamber and a preheating chamber therein above said secondcombustion chamber, a second partition interposed between said secondcombustion chamber and said preheating chamber and having passagestherethrough, means for withdrawing gas from said reaction chamber andsupplying it to said second combustion chamber for combustion therein tosupply heat and combustion gases to said preheating chamber to preheatand fluidize a reducible material therein, means for discharging saidpreheated material from said preheating chamber to said reaction zone,and means for discharging said at least partially reduced material fromsaid reaction zone.

2. The apparatus set forth in claim 1 comprising a melting furnaceconnected with the means for discharging the material from said reactionzone.

3. The apparatus set forth in claim 1 comprising a melting furnaceconnected with the means for discharging the material from said reactingzone and a separator interposed in the means for withdrawing gas fromsaid reaction chamber and communicating with said furnace for separatingfinely divided at least partially reduced material from the gas andsupplying it to said furnace.

4. The apparatus set forth in claim 1 comprising an electric arc furnacehaving electrodes and a stack eXtend ing above said electrodes andconnected with the means for discharging material from said reactionzone to provide a column of said material for feeding to saidelectrodes.

5. The apparatus set forth in claim 4 in which the walls of said stackdiverge downwardly.

6. The apparatus set forth in claim 1 comprising an electric arc furnacehaving a melting chamber, electrodes extending downwardly into saidmelting chamber, and a stack extending above said electrodes andchamber, and connected with the means for discharging material from saidreaction zone to provide a column of said material for feeding to saidelectrodes.

7. The apparatus set forth in claim 1 comprising a melting furnaceconnected with the means for discharging the material from said reactingzone, said furnace having electrodes and a stack extending above saidelectrodes for maintaining a head of said at least partially reducedmaterial for feeding said electrodes, and a separator interposed in themeans for withdrawing gas from said reaction chamber and communicatingwith said stack for separating finely divided at least partially reducedmaterial from the gas and supplying it to said furnace.

8. A process for the production of a metal from finely divided particlesof a reducible compound thereof, which comprises preheating saidparticles in a preheating zone, conducting said preheated particles to areducing zone wherein said particles are suspended in a stream ofreducing gas at a temperature and for a time sufficient to reduce themajor part of said particles to metal, the conditions in said reducingzone being those of a fluidized reducing bed, introducing gases emergingfrom said reducing zone into a combustion zone intermediate saidpreheating and reducing zones, burning said gases in said combustionzone and introducing the combustion products into said preheating zoneto heat the particles therein and thereafter consolidating the hotreduced particles into molten metal while maintaining the particlesout-of-contact with the atmosphere.

9. A process for the production of a metal from finely divided particlesof a reducible compound thereof, which comprises preheating saidparticles in a preheating zone, conducting said preheated particles to areducing zone wherein said particles are suspended in a stream ofreducing gas at a temperature and for a time suflicient to reduce themajor part of said particles to metal, the conditions in said reducingzone being those of a fluidized reducing bed, introducing the gasesemerging from the reducing zone into a combustion zone intermediate saidpreheating and reducing zones, burning said gases in said combustionzone and introducing the combustion products into the preheating zone toheat said particles therein, and thereafter consolidating the hotreduced particles while maintaining the particles out-of contact withthe atmosphere.

References Cited in the file of this patent UNITED STATES PATENTS2,368,508 Wile Jan. 30, 1945 2,477,454 Heath July 26, 1949 2,481,217Hemminger Sept. 6, 1949 2,638,414 Lewis May 12, 1953 2,752,234 ShipleyJune 26, 1956

8. A PROCESS FOR THE PRODUCTION OF A METAL FROM FINELY DIVIDED PARTICLESOF A REDUCIBLE COMPOUND THEREOF, WHICH COMPRISES PREHEATING SAIDPARTILCES IN A PREHEATING ZONE CONDUCTING SAID PREHEATED PARTICLES TO AREDUCING ZONE WHEREIN SAID PARTICLES ARE SUSPENDED IN A STREAM OFREDUCING GAS AT A TEMPERATURE AND FOR A TIME SUFFICIENT TO REDUCE THEMAJOR PART OF SAID PARTICLES TO METAL, THE CONDITIONS IN SAID REDUCINGZONE BEING THOSE OF A FLUIDED REDUCING BED, INTRODUCING GASES EMERGINGFROM SAID REDUCING ZONE INTO A COMBUTION ZONE INTERMEDIATE SAIDPREHEATING AND REDUCING ZONES, BURNING SAID GASES IN SAID COMBUSTIONZONE AND INTRODUCING THE COMBUSTION PRODUCTS INTO SAID PREHEATING ZONETO HEAT THE PARTICLES THEREIN